Although Si is a typical semiconductor material, SiC, GaN, and the like also have been researched as a semiconductor material in addition to Si. However, SiC, GaN, and the like need to be subjected to treatment at a temperature higher than or equal to 1500° C. so as to be crystallized and used as a single crystal component; therefore, these semiconductor materials cannot be used for thin film devices or three-dimensional devices.
On the other hand, in recent years, techniques to form transistors using a semiconductor thin film (with a thickness of approximately several nanometers to several hundreds of nanometers) which is formed over a substrate having an insulating surface at a relatively low temperature have attracted attention. The transistors are widely applied to electronic devices such as an IC and an electro-optical device, and development thereof as switching elements for an image display device has been particularly expected.
There are various kinds of metal oxides, which are used for a wide range of applications. Indium oxide is a well-known material and is used as a light-transmitting electrode material which is necessary for liquid crystal displays and the like. Some metal oxides have semiconductor characteristics. Examples of the metal oxides having semiconductor characteristics are tungsten oxide, tin oxide, indium oxide, zinc oxide, and the like. Transistors in which a channel formation region is formed using such a metal oxide having semiconductor characteristics are already known (Patent Document 1 and Patent Document 2).